Does your breathing seem heavier at the start than in the middle or at the end of your run?
Fear not. The body is an integrated system that kicks into gear as you start your run and increase the oxygen demands of your muscles. The regulation system increases your heart rate to pump more oxygen carrying blood to the working muscles and your respiratory rate to exchange more oxygen and carbon dioxide between the blood and air.
When you're not exercising, the muscle that does most of the work is the diaphragm, located below the lung cavity and above the abdominal cavity. As the diaphragm contracts, air is sucked into the lungs.
As you begin to exercise, the accessory muscles of breathing become more active, lifting the rib cage up to create more suction and increasing the amount of air that is available to exchange. This complements the work of the diaphragm and contributes to the appearance of the chest heaving during heavy breathing during and after exercise.
At the start of your run, the breathing muscles are beginning the warm-up phase which will give you the “heavy breathing” feeling. When all the muscles are primed and working in together, that sensation goes away. Starting slow and warming up for the early phase of your run, or before you pick up speed, may reduce the heavy sensation.
A word of caution: Warm-up angina (heart pain) may also present with chest heaviness. I would characterise it as a pressure below the breastbone associated with the start of exercise or an inability to get to speed as you begin your run that gets better as your run progresses. It may be not always be easy to tell the difference between “heavy breathing” and “warm-up angina.” If you have any doubts, you should see your doctor to discuss additional testing.